The biochemical interactions among all types of plants, including microorganisms. The term is usually interpreted as the detrimental influence of one plant upon another but is used more and more, as intended originally, to encompass both detrimental and beneficial interactions. At least two forms of allelopathy are distinguished: (1) the production and release of an allelochemical by one species inhibiting the growth of only other adjacent species, which may confer competitive advantage for the allelopathic species; and (2) autoallelopathy, in which both the species producing the allelochemical and unrelated species are indiscriminately affected. The term allelopathy, frequently restricted to interactions among higher plants, is now applied to interactions among plants from all divisions, including algae. Even interactions between plants and herbivorous insects or nematodes in which plant substances attract, repel, deter, or retard the growth of attacking insects or nematodes are considered to be allelopathic. Interactions between soil microorganisms and plants are important in allelopathy. Fungi and bacteria may produce and release inhibitors or promoters. Some bacteria enhance plant growth through fixing nitrogen, others through providing phosphorus. The activity of nitrogen-fixing bacteria may be affected by allelochemicals, and this effect in turn may influence ecological patterns. The rhizosphere must be considered the main site for allelopathic interactions.Nitrogen fixation Rhizosphere
Allelopathy is clearly distinguished from competition: In allelopathy a chemical is introduced by the plant into the environment, whereas in competition the plant removes or reduces such environmental components as minerals, water, space, gas exchange, and light. In the field, both allelopathy and competition usually act simultaneously.